1. Field of the Invention
The present invention relates to the in-band transmission of TTY (teletypewriter) signals through wireless systems, and more specifically to the in-band transmission of TTY signals through wireless systems that employ low bit-rate voice compression.
2. Discussion of the Background
Recently, FCC has mandated that US wireless carriers to provide E911 capability to the hearing and speech impaired by enabling this capability through interfaces with TTY/TDD terminals. To meet this mandate, TIA TR41 had assembled an ad hoc committee on Telecommunications Devices for the Deaf (TDD). This ad hoc committee has drafted the specifications for all TTY/TDD terminals for which this FCC mandate applies. These specifications are listed in EIA standards PN-1663, “Telecommunications Devices for the Deaf,” January 1985, and are a subset of ITU-T Recommendation v.18, which is incorporated herein by reference.
The EIA standards document requires all TTY/TDD terminals covered by the FCC mandate to provide Baudot code operation employing half-duplex asynchronous transmission. Baudot operation employs frequency shift keying techniques to encode 0's (denoted as SPACE) and 1's (denoted as MARK). The frequency 1400 Hz is used for denoting MARK and 1800 Hz is used for denoting SPACE. Every bit is represented by either the MARK or SPACE frequency for a duration of 22 ms. The set of 5-bit Baudot codes is listed in EIA standards PN-1663. Both letters as well as figures are accommodated in this list by utilizing specific shift codes. Each 5-bit TTY character is encoded using the format shown in Table 1.
TABLE 1Format of TTY characterBitStartDataDataDataDataDataStopSignalSpaceLSBBit 2Bit 3Bit 4MSBMark#Bits1111111.5–2.0Note that data bits are encoded as SPACE if they are 0 and as MARK if they are 1. The nominal transmit level for the TTY signal is −10 dBm. Finally, at the end of the TTY session a MARK hold tone is transmitted for an additional period of 150–300 ms.
The testing of systems that support TTY/TDD terminals for cellular and PCS carriers was addressed by Lober and Walsh who established an industry standard test procedure. This test procedure is described in Lober and Walsh Engineering, Inc.—Cellular Product Technologies, LLC, NENA/BellSouth Technologies, Inc., “TTY Over Cellular Laboratory and Field Test Procedure,” September 1998, which is incorporated herein by reference. Briefly, this test procedure generates random character files. These source character files are subsequently converted to an audio signal by a TTY/TDD generator terminal and transmitted over the system under test. The received audio files are fed to a TTY/TDD receiver terminal which converts them into an output character file. The Lober and Walsh test procedure compares the output character file with the source character file and presents the results as TCER (total character error rate) as well as PCER (printable character error rate). Currently, the consensus in the GTF or CDG bodies is that in order to meet the FCC mandate for cellular and PCS carriers, the TCER must be less than 1% across the cell for all operating conditions, i.e., above 16 dB C/N for downlink and 18 dB C/N for uplink.
In wireless systems that employ high bit rate codecs (16 Kbps and above), the in-band transmission of these TTY signals may not present a problem. However for systems that employ low bit-rate voice codecs, the FCC mandate presents a hurdle.